Global warming was once an uncommon term used by a few scientists who were growing concerned over the effects of decades of pollution on long-term weather patterns. Today, the idea of global warming is well known, if not well understood. It is not unusual to hear someone complaining about a hot day or a freak storm and remark, "It's global warming."

Well, is it? In this article, we'll learn what global warming is, what causes it and what the effects could be. Not everyone is convinced that global warming is happening; and even if it is happening, some aren't sure it's something we need to worry about.

So what is global warming and how does it happen? To understand the global warming phenomenon, first we have to learn about the difference between weather and climate.

Weather and Climate
Weather is local and short-term. If it snows in the town where you live next Tuesday, that's weather. Climate is long-term and doesn't relate to one small location. The climate of an area is the average weather conditions in a region over a long period of time. If the part of the world you live in has cold winters with lots of snow, that would be part of the climate for the region you live in. The winters there have been cold and snowy for as long as weather has been recorded, so we know generally what to expect.

Courtesy NOAA Mean temperature for December through February in the United States from 1895-2005, indicating climate

It's important to understand that when we talk about climate being long-term, we mean really long-term. Even talking about a few hundred years is pretty short-term when it comes to climate. In fact, changes in climate sometimes take tens of thousands of years. That means if you happen to have a winter that isn't as cold as usual, with not very much snow -- or even two or three such winters in a row -- that isn't a change in climate. That's just an anomaly -- an event that falls outside of the usual statistical range but doesn't represent any permanent, long-term change.

It's also important to understand that even small changes in climate can have major effects. When scientists talk about "the Ice Age," you probably envision the world frozen, covered with snow and suffering from frigid temperatures. In fact, during the last ice age (ice ages recur roughly every 50,000 to 100,000 years), the earth's average temperature was only 5 Celsius degrees cooler than modern temperature averages.

Global warming is a significant increase in the Earth's climatic temperature over a relatively short period of time as a result of the activities of humans.

In specific terms, an increase of 1 or more Celsius degrees in a period of one hundred to two hundred years would be considered global warming. Over the course of a single century, an increase of even 0.4 degrees Celsius would be significant.

Natural Changes in Climate It can take the Earth thousands of years to warm up or cool down just 1 degree when it happens naturally. And it does happen naturally. In addition to recurring ice age cycles, the Earth's climate can change because of volcanic activity, differences in the plant life that covers much of the planet, changes in the amount of radiation the sun gives off and natural changes in the chemistry of the atmosphere.

The Greenhouse Effect
Global warming is caused by an increase in the greenhouse effect. The greenhouse effect is not a bad thing by itself -- it's what allows Earth to stay warm enough for life to survive.

You can think of the Earth sort of like your car sitting out in a parking lot on a sunny day. You've probably noticed that your car is always much hotter inside than the outside temperature if it's been sitting there for a while. The sun's rays enter through your car's windows. Some of the heat from the sun is absorbed by the seats, the dashboard and the carpeting and floor mats. When those objects release this heat, it doesn't all get out through the windows. Some is reflected back in -- the heat radiated by the seats is a different wavelength than the light of the sun that made it through the windows in the first place. So a certain amount of energy is going in, and less energy is going out. The result is a gradual increase in the temperature inside your car.

Courtesy U.S. Global Change Research Program

When the sun's rays hit the Earth's atmosphere and the surface of the Earth, approximately 70 percent of the energy stays on the planet, absorbed by land, oceans, plants and other things. The other 30 percent is reflected into space by clouds, snow fields and other reflective surfaces [ref]. But even the 70 percent that gets through doesn't stay on earth forever (otherwise the Earth would become a blazing fireball). The things around the planet that absorb the sun's heat eventually radiate that heat back out. Some of it makes it into space, and the rest of it ends up getting reflected back down to earth when it hits certain things in the atmosphere, such as carbon dioxide, methane gas and water vapor. The heat that doesn't make it out through Earth's atmosphere keeps the planet warmer than it is in outer space, because more energy is coming in through the atmosphere than is going out. This is all part of the greenhouse effect that keeps the Earth warm.

Earth Without the Greenhouse Effect What would Earth look like if there weren't any greenhouse effect at all? It would probably look a lot like Mars. Mars doesn't have a thick enough atmosphere to reflect enough heat back to the planet, so it gets very cold there. Some scientists have suggested that we could terraform the surface of Mars by sending "factories" that would spew water vapor and carbon dioxide into the air. If enough material could be generated, the atmosphere might start to thicken enough to retain more heat and allow plants to live on the surface. Once plants spread across Mars, they would start producing oxygen. After a few hundred or thousand years, Mars might actually have an environment that humans could simply walk around in -- all thanks to the greenhouse effect.

Global Warming: What's Happening?
The greenhouse effect happens because of certain naturally occurring substances in the atmosphere. Unfortunately, since the Industrial Revolution, humans have been pouring huge amounts of those substances into the air.

Courtesy NASA Power plants, cattle and cars are some of the major contributors of greenhouse gases such as carbon dioxide and methane.

Carbon dioxide (CO2) is a colorless gas that is a by-product of the combustion of organic matter. It makes up less than 0.04 percent of Earth's atmosphere, most of which was put there by volcanic activity very early in the planet's life. Today, human activities are pumping huge amounts of CO2 into the atmosphere, resulting in an overall increase in carbon dioxide concentrations. These increased concentrations are considered the primary factor in global warming, because carbon dioxide absorbs infrared radiation. Most of the energy that escapes Earth's atmosphere comes in this form, so extra CO2 means more energy absorption and an overall increase in the planet's temperature.

Courtesy NOAA, Dave Keeling and Tim Whorf (Schipps Institution of Oceanography) Carbon dioxide concentration as measured at Mauna Loa, Hawaii

The Worldwatch Institute reports that carbon emissions worldwide have increased from about 1 billion tons in 1900 to about 7 billion tons in 1995. The Institute also notes that the average surface temperature of Earth has gone from 14.5 degrees C in 1860 to 15.3 degrees C in 1980.

Nitrous oxide (NO2) is another important greenhouse gas. Although the amounts being released by human activities are not as great as the amounts of CO2, nitrous oxide absorbs much more energy than CO2 (about 270 times as much). For this reason, efforts to curb greenhouse gas emissions have focused on NO2 as well. The use of large amounts of nitrogen fertilizer on crops releases nitrous oxide in great quantities, and it is also a by-product of combustion.

Methane is a combustible gas, and it is the main component of natural gas. Methane occurs naturally through the decomposition of organic material and is often encountered in the form of "swamp gas." Man-made processes produce methane in several ways:

• By extracting it from coal
• From large herds of livestock (i.e., digestive gases)
• From the bacteria in rice paddies
• Decomposition of garbage in landfills

Water Vapor, The Other Greenhouse Gas Water vapor is the most abundant greenhouse gas, but it is more often than not a result of climate changes rather than man-made emissions. Water or moisture on the Earth's surface absorbs heat from the sun and the surroundings. When enough heat has been absorbed, some of the liquid's molecules may have enough energy to escape from the liquid and begin to rise into the atmosphere as a vapor. As the vapor rises higher and higher, the temperature of the surrounding air becomes lower and lower. Eventually, the vapor loses enough heat to the surrounding air to allow it to turn back into a liquid. Earth's gravitational pull then causes the liquid to "fall" back down to the earth, thereby completing the cycle. This cycle is also called a "positive feedback loop." Water vapor is more difficult to measure than the other greenhouse gases and scientists are uncertain as to the exact part that it plays in global warming. But, the NOAA Web site does have this to say: As water vapor increases in the atmosphere, more of it will eventually also condense into clouds, which are more able to reflect incoming solar radiation (thus allowing less energy to reach the Earth's surface and heat it up).

What will actually happen if the entire planet warms up a few degrees? Read the next section to find out.

Effects of Global Warming: Sea Level
We have seen that an average drop of just 5 degrees Celsius over thousands of years can cause an ice age; so what will happen if the Earth's average temperature increases a few degrees in just a few hundred years? There is no clear answer. Even short-term weather predictions are never perfectly accurate because weather is a complex phenomenon. When it comes to long-term climate predictions, all we can manage are educated guesses based on our knowledge of climate patterns through history.

Photo courtesy NASA Some possible effects of global warming are the inundation of low-lying islands due to rising sea levels, increased frequency of severe storms and the retreat of glaciers and icecaps.

Glaciers and ice shelves around the world could begin to melt. In fact, this is already happening. The loss of large areas of ice on the surface could accelerate global warming because less of the sun's energy would be reflected away from Earth to begin with (refer back to our discussion of the greenhouse effect). An immediate result of melting glaciers would be a rise in sea levels. Initially, this would only be an inch or two (however, if the West Antarctic Ice Sheet were to melt and collapse into the sea, it would push sea levels up 10 feet, and many coastal areas would completely disappear beneath the ocean). Sea levels would also rise because ocean waters would grow warmer, causing the water to expand. Even a modest rise in sea levels could cause flooding problems for low-lying coastal areas.

Photo courtesy Earth Observatory, NOAA Research predictions indicate a rising sea level.

With a rise in the overall temperature of the ocean, ocean-borne storms such as tropical storms and hurricanes, which get their fierce and destructive energy from the warm waters they pass over, would increase in number and force.

Effects of Global Warming: Seasons and Ecosystems
Less abrupt changes would occur around the world as average temperatures increased. In temperate areas with four seasons, the growing season would be longer with more precipitation. This could be beneficial in many ways for these areas. However, less temperate parts of the world would likely see an increase in temperature and a sharp decrease in precipitation, causing long droughts and potentially creating deserts.

The most devastating effects, and also the hardest to predict, would be the effects on the world's living ecosystems. Many ecosystems are very delicate, and the slightest change can kill off several species as well as any other species that depend on them. Most ecosystems are interconnected, so the chain reaction of effects could be immeasurable. The results could be something like a forest gradually dying off and turning to grassland or entire coral reefs dying. Many species of plants and animals would adapt or move to deal with the shift in climate, but many would become extinct.

The human cost of global warming is hard to quantify. Thousands of lives per year could be lost as the elderly or ill suffer from heat stroke and other heat-related trauma. Poor people and underdeveloped nations would suffer the worst effects, since they would not have the financial resources to deal with the problems that come with an increase in temperature. Huge numbers of people could die from starvation if a decrease in precipitation limits crop growth and from disease if coastal flooding leads to widespread water-borne illness.

Next, we'll find out why some people aren't concerned about global warming.

Is Global Warming a Real Problem?
Some people don't think global warming is happening at all. There are several reasons for this:

• They don't think the data show a measurable upward trend in global temperatures, either because we don't have enough long-term historical climate data or because the data we do have isn't clear enough.

• Some scientists think that data is being interpreted incorrectly by people who are already worried about global warming. That is, these people are looking for evidence of global warming in the statistics, instead of looking at the evidence objectively and trying to figure out what it means.

• Any increase in global temperatures we are seeing could be a natural climate shift, or it could be due to other factors than greenhouse gases.

What's the correct answer? It can be hard to figure out. Most scientists will tell you that global warming is real and that it is likely to do some kind of harm, but the extent of the problem and the danger posed by the effects are wide open for debate.

In the next section, we'll see if there's anything we can do to help prevent global warming.

Can We Stop Global Warming?
There are a few things we can do to try and halt global warming. Basically, they all boil down to this: Don't use as much of the stuff that creates greenhouse gases. On a local level, you can help by using less energy. The electricity that operates many of the devices in our homes comes from a power plant, and most power plants burn fossil fuels to generate that power. Turn off lights when they're not in use. Take shorter showers to use less hot water. Use a fan instead of an air conditioner on a warm day.

Courtesy BMW AG, Munich, Germany; DOE/NREL; SunLine Transit Agency Hydrogen-powered cars, the increased use of solar cells, and hydro-electric power plants are possible ways to reduce the emission of greenhouse gases.

Here are some other specific ways you can help decrease greenhouse-gas emissions:

• Make sure your car is properly tuned up. This allows it to run more efficiently and generated fewer harmful gases.
• Walk or ride a bike when you can. Driving your car generates more greenhouse gases than almost anything else you do.
• Turn lights and other appliances off when you're not using them. Even though a light bulb doesn't generate greenhouse gas, the power plant that generates the electricity used by the light bulb probably does.
• Recycle. Garbage that doesn't get recycled ends up in a landfill, generating methane; plus, recycled goods require less energy to produce than products made from scratch.
• Plant trees and other plant life where you can. Plants take carbon dioxide out of the air and release oxygen.
• Don't burn garbage. This releases carbon dioxide and hydrocarbons into the atmosphere.

To really stem the emission of greenhouse gases, we need to develop non-fossil fuel energy sources. Hydro-electric power, solar power, hydrogen engines and fuel cells could all create big cuts in greenhouse gases if they were to become more common.

At the international level, the Kyoto treaty was written to reduce CO2 and other greenhouse gas emissions worldwide. Thirty-five industrialized nations have committed to reducing their output of those gases to varying degrees. Unfortunately, the United States, the world's primary producer of greenhouse gases, did not sign the treaty.